There have been many attempts to develop commercially viable systems to harness the energy from ocean waves. None have yet succeeded in doing so. A successful and economically viable wave energy system must be both robust and efficient. It must be correctly matched to the variety of wave climates that it will be expected to operate in during its lifetime and it must have a sufficiently high power to weight ratio to justify the considerable cost of transportation, installation and deployment.
In general, a wave energy system must by necessity comprise at least two bodies. The first of these is the Wave Actuated Body (WAB) which is the body that is primarily subject to the influence of the waves, the second is a reactive body to which reference is made as power is removed from the system. A wave energy system must also include a power take-off (PTO) system that converts the mechanical energy in the relative motion of the two bodies into a more suitable (or higher valued) form for human consumption.
A wave energy system is ultimately actuated by dynamic pressure and velocity gradients within the water column. These gradients oscillate with the incident wave period which for a typical northern hemisphere location might be in the region of 8 to 12 seconds. Wave Energy systems are therefore typically characterised by low speed and high load reciprocating motions.
A PTO within a wave energy system will, ideally, convert the high load, low speed input motion into a continuous flow of electricity, potable water, hydrogen, heat or any other suitable product than can be sold for commercial advantage. The PTO system will typically require one or more intermediary stages which may be mechanical, hydraulic, pneumatic or electrical and the system includes the necessary conversion and transmission components to facilitate the conversion process.
Hydraulic power take-off systems are a common feature of Wave Energy Converter designs due to their high load, low speed operating characteristics and high power to weight ratio. Examples of wave energy systems utilising hydraulic PTO include patent documents WO 2004/007953, GB2467011A and [WO 2004/088129 A1.
In addition, the design of a viable wave energy system is complicated by the need to ensure that the mechanism does not encounter an end stop during any anticipated operation. A mechanical hard stop (or end stop) will result in high structural loading and the potential for catastrophic failure if it acts to restrain the free motion of the device. As it is difficult to predict the maximum wave that a given site will experience during the lifetime of a device end stops should be avoided where possible.
The design of a wave energy system must also use either a rotating or linear conversion device. While a rotating system is essentially unconstrained by the ‘end stop’ problem, motions will typically be reciprocating and of low amplitude. Conventional rotating machinery is generally designed for high speed operation and unsuitable for this application.
One common approach as described in patent document GB2467011A is to locate a crank arm on a rotating joint and use a linear conversion mechanism. While this solution deals with the end stop problem, it necessarily limits the useful stroke of the ram to twice the length of the crank arm. In operation the device will typically only utilise a fraction of this available stroke due to the small rotational amplitude.
A wave energy device will typically be constrained by the load that the bearings can withstand. In seeking to increase the power transmitted through a hydraulic ram the designer has the option of increasing the pressure of the hydraulic system, the bore of the ram or the stroke of the ram. The first two of these options will necessarily increase the bearing load and ultimately constrain the power output of the device. At this point more power can only be delivered by increasing the active stroke length of the ram.
A further issue in wave energy is the production of high quality power—(which may be defined as a smooth and consistent flow of power). The primary conversion mechanism in the power conversion train of a wave energy converter will typically encounter cyclic loading and will therefore result in pulses of energy delivered at the incident wave frequency. By incorporating double acting components into the primary conversion mechanism it is possible to increase the apparent frequency at which the power is delivered to the secondary conversion device. The delivered power can be further smoothed by incorporating correctly sized accumulators, or energy storage devices, between the primary and secondary conversion systems. Many developers of wave energy converters have adopted this approach as disclosed in patent documents U.S. 2008/0231054 A1 and U.S. 2008/0191485 A1.
An enhanced means of providing a smooth flow of power is shown in patent document GB0900685.9 where the wave field is sampled at numerous points to provide phase shifting and a continuous flow of power. This patent describes the use of a tri-axial node member that allows energy to be captured from out of plane motions and delivers a local phase difference between three primary PTO components collocated at a single nodal point.